There are many power-consuming devices designed to be implanted in the body of a human. Such devices frequently include a power source, such as a battery, that must be periodically recharged for the device to remain functional. Alternatively or additionally, an implantable device may receive operational power from an external charging system, for example, via an inductive charging circuit. For example, U.S. Patent Application Publication No. US 2012/0209165 A1 to Degen et al., assigned to the assignee of the present application, describes an example in which an implantable device, including an electro-mechanical pump is powered by a rechargeable battery, which is periodically recharged via an inductive charging circuit.
In the system described in the foregoing publication, energy is transmitted to a receiving circuit disposed within the implant by magnetically coupling a transmitting coil in an external charging system to a receiving coil in the implantable device. An alternating current flowing in the transmitting coil induces an alternating current to flow in the receiving coil. The current in the receiving coil is converted to a form suitable for recharging a battery disposed within the implantable device, or in some cases directly powering the electro-mechanical pump.
As described in the foregoing application, circuitry within the implantable device may heat up in response to the current flowing through the receiving coil or the voltage built up across the receiving coil, causing damage to the electromechanical components and circuitry disposed within the implantable device. Specifically, such heating may cause deterioration of the circuitry in the implantable device, or increased wear in mechanical components of the implantable device due to reduced clearances between components. Heating also may cause degradation of a humidity barrier over implant circuitry, thereby allowing moisture into the circuitry, possibly causing improper performance or implant damage. In addition, if the temperature of the circuitry increases too much, excessive heat may be transferred to the tissue surrounding the implantable device, causing discomfort or injury to that tissue.
In the system described in the foregoing application, the implantable device includes a temperature sensor disposed to monitor the battery temperature and a radio transceiver configured to transmit battery temperature data to the external charging system. A controller located within the external charging system is programmed to analyze the battery temperature reported by the implantable device, and to adjust the charging power supplied to inductive circuit of the external charging system to maintain the temperature of the implantable device below a predetermined threshold, e.g., less than 2° C. above body temperature. In one embodiment, the power supplied to the inductive coil of the external charging system is cycled between high power (e.g., 120 mA) and low power (e.g., 40 mA) charging intervals responsive to the measured temperature within the implantable device.
While the system described in the foregoing application effectively limits temperature transients experienced by the receiving circuit within the implantable device, it requires the use of the radio transceiver as a separate communications path to transmit temperature information to the external charging system, which information is in turn processed to intermittently reduce the power supplied to inductive circuit.
In view of the complexity of the inductive charging system described in the foregoing application, it would be desirable to provide an inductive charging system for an implantable device that directly regulates energy absorption of the receiving circuit of the implantable device, without the need for a separate communications path to an external charging system.
It further would be desirable to provide an inductive charging system for an implantable device that is capable of limiting temperature excursions within the receiving circuit of the implantable device by directly regulating energy absorption of the receiving circuit in real-time, without a time lag associated with transmission and analysis of data from the implantable device to an external charging system.
It still further would be desirable to provide circuits and methods for regulating energy absorption by the receiving circuit of an implantable device that reduce generation of ohmic heating within the receiving circuit.